This application claims priority under 35 U.S.C. xc2xa7xc2xa7119 and/or 365 to Appln. No. 199 58 600.4 filed in Germany on Dec. 6, 1999; the entire content of which is hereby incorporated by reference.
1. Technical field
The invention relates to a method for producing a fiberoptic waveguide, and to a fiberoptic waveguide having a basic segment and a phase shift segment in accordance with the preamble of patent claims 1 and 10.
2. Prior art
A fiberoptic waveguide of the generic type and having a basic segment and a phase shift segment is disclosed, for example, in EP-A-0,856,737, where it is used in a magnetooptic current sensor. This waveguide has two polarization-maintaining basic fibers with elliptical cores, said fibers being called the feeder fiber and return fiber, and a sensor fiber which has a round core, is arranged between these two fibers, and is wound in the form of a coil around an electric conductor. Present as transitional elements between the basic fibers and sensor fiber is one phase shift element each in the form of a fiberoptical xcex/4 time-delay element whose likewise elliptical core is rotated by 45xc2x0 with respect to the core of the basic fibers. Linearly polarized waves which propagate in the feeder fiber are decomposed upon transition into the time-delay element into two orthogonal polarization components which are aligned parallel to the main axes of the core of the time-delay element. The length of the xcex/4 time-delay element is selected in this case such that the two polarization components at its end have an optical phase difference of 90xc2x0 because of the birefringence. The light emerging from the time-delay element is then circularly polarized. Consequently, it is possible to use a first time-delay element to generate from linearly polarized waves of the feeder fiber circularly polarized waves which can propagate in the sensor fiber and are converted back again by a second time-delay element into linearly polarized waves whose polarization is parallel to a main axis of the elliptical core of the return fiber, such that they can propagate in the latter.
The production of such a fiberoptic waveguide requires some skill. In accordance with the prior art, the first step for this purpose is to align the elliptical cores of the basic fiber and of the time-delay element with an orientation of 45xc2x0 relative to one another. The alignment is performed by means of polarizers and is very time consuming as a rule. Thereafter, one fiber end each of the basic fiber and of the time-delay element are connected to one another, this being performed as a rule by means of arc welding, also known as splicing.
It is true that splicers are known which automatically determine the angular orientation of fiber cores by lateral transirradiation of the fibers. This method delivers good results for fibers with stress-induced birefringence. However, it is inadequate for the above-described fibers, whose birefringence is based on an elliptical core. This applies, in particular, in the case of fibers which are designed for small wavelengths of at most 850 nm, since the elliptical fiber cores are very small and the splicer cannot detect their orientation with sufficient accuracy.
It is therefore an object of the invention to create a method for producing a fiberoptic waveguide of the type mentioned at the beginning, as well as a waveguide, which method and waveguide permit simple production even for fibers with elliptical cores.
This object is achieved by a method having the features of patent claim 1, and by a waveguide having the features of patent claim 10.
In the method according to the invention, an optical fiber is twisted by a defined angle and a zone of the fiber is heated until the torsion in this zone is released so as to produce adjacent to this zone a basic segment on one side and, on the other side, a phase shift segment or a segment with an alignment corresponding to the phase shift segment to be achieved.
A complicated alignment of two fibers is superfluous, since it is replaced by a simple torsion about an optical axis or longitudinal axis of a fiber. The torsion is released by local heating of the fiber material. There is produced inside the fiber a stress-relief zone which subdivides the fiber into two segments whose fiber cores are aligned relative to one another by the torsion angle.
It is possible thereby to create a fiberoptic waveguide which has in a single-piece optical fiber both a basic segment and a phase shift segment, the basic segment and phase shift segment having cores which are aligned relative to one another by a defined angle, and the two segments being separated from one another by the heated and re-solidified stress-relief zone. The cores of the basic segment and the shift segment have the same form, specifically that of the core of the optical fiber.
In a first variant of the method, the optical fiber is connected to a second optical fiber, both fibers being twisted with one another. The stress-relief zone is selected in this case such that it is situated at a defined distance from a joint of the two fibers.
In another variant of the method, only the optical fiber is twisted, the position of the stress-relief zone being selected arbitrarily. Only after solidification of this zone is the optical fiber broken at a defined distance therefrom in order to form a phase shift segment with the length it requires. Subsequently, a second fiber, in particular having a different core, can be spliced to this break.
It is advantageous that it is possible in the method according to the invention to achieve fine correction of the state of polarization and/or of the phase shift segment by setting the birefringence of the phase shift segment by heating.
Further advantageous embodiments follow from the dependent patent claims.